This invention relates to clear, impact modified resins, and more particularly to clear impact modifiers for thermoplastic resins. Still more particularly, this invention relates to impact modifiers for use in producing clear, impact modified polyvinyl chloride (PVC) formulations.
Polyvinyl chloride resins are well known as thermoplastic molding resins, and have achieved wide acceptance commercially for a great variety of applications. When unplasticized or only slightly plasticized, PVC resins are generally quite brittle; the use of rubbery impact modifiers to overcome this deficiency has been practiced for over thirty years. Typical of the impact modifiers developed for this purpose are those based on graft copolymers, as shown for example in U.S. Pat. No. 3,073,798.
An additional property of PVC that is important for many applications is its inherent clarity and transparency. Many of the impact modifiers initially developed for use with PVC greatly reduced clarity and transparency, and considerable effort has been devoted to developing improved impact modifiers that do not exhibit this deficiency. A variety of methods for preparing clear PVC impact modifiers have since been developed, and a great many such modifiers are now commercially available. The method most frequently used is based on matching the apparent refractive index of the impact modifier to that of PVC. For example, the method taught in U.S. Pat. No. 3,657,390 requires the use of two different graft copolymers, one having a refractive index above that of PVC and the second having a refractive index below PVC, in an appropriate ratio. In U.S. Pat. No. 3,636,138, a complex preparation of a two-stage graft copolymer is set forth wherein careful selection of the grafting monomers in each stage is used to adjust the apparent refractive index of the resulting composite graft copolymer. In U.S. Pat. No. 3,657,391 there is disclosed a clear impact modifier prepared by grafting a diene-vinylaromatic rubber substrate having a refractive index matching PVC with a narrowly specified combination of grafting monomers also having a refractive index matching PVC.
According to these references, the adjustment and matching of refractive index for these impact modifiers has depended in part upon the use of acrylate monomers in the grafting phase. As pointed out in U.S. Pat. No. 3,636,138, grafting a rubber substrate having a refractive index to one side of that of PVC with a monomer mixture having a refractive index to the other side of PVC to achieve the requisite matching is known, and alkyl acrylates, particularly methyl methacrylates are most often used for this purpose. However, the presence of acrylate monomers, and particularly methacrylates, is said to reduce optimum adhesion to PVC, and the requisite clarity is thus achieved with some compromise of other properties.
Acrylonitrile-butadiene-styrene (ABS) graft copolymers have long been known as excellent impact modifiers for PVC resins. However, these ABS resins have generally not been useful where superior clarity and color of the impact modified composition is important. According to the teachings of the references, clarity is only accomplished with ABS compositions that do not give good impact modification.
A method for producing clear ABS impact modifiers, particularly for use in PVC compositions, would thus be a useful advance in the compounding art.